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Monday, 10 December 2012

Complete Heart Block with Narrow QRS escape Rhythm

A new ECG is launched most Monday nights, in term time. Cases are generally aimed at Primary Care.

Please use the Hashtag #ECGclass if you want to ask the patient any questions, or request any further investigations.

Remember there are no right or wrong answers! ECG interpretation is often open to debate, and will usually evolve as new information becomes available. Everyone's opinion is valid, and useful to others, towards this evolution process.

Please feel free to join in, but most important of all, have fun! :-)

#ECGclass Case 19

Today's case is adapted from old ECGclass, from the early (pre-blog) days.

So those of you who were in from the beginning have no excuses! :)A 74yrs gentleman comes to see you with funny turns. His wife describes him as having brief episodes of transient 'faint-like' attacks, when he seems unresponsive for a moment, and looks very pale. On one occasion he actually collapsed, but came round moments later. Obviously - you want an ECG - this is #ECGclass after all!So here it is:

Q1. Is the rhythm Regular or Irregular? Q2. Can you see P-waves? Q3. What is the rate? (Atrial? and Ventricular?)Q4. Does the P-QRS morphology look normal, or abnormal?Update 1So far, having answered Questions 1 to 4, you can already make the following comments (Remember the art of ECG bluffing - July blog) :This is a 12 lead ECG, on which electrical activity can be seen.The gain calibration is noted and normal, but no paper-speed is noted. For calculation purposes, we will assume standard paper speed of 25mm/sec.When mapped out, the ventricular rate is regular at 50 beats per minute. (6 large squares between complexes = 300/6). The Atrial rate is also regular at around 85 beats per minute. (somewhere between 3 and 4 large squares between complexes = 300/3.5)P waves, which are usually best seen in V1 and V2 , are not clearly seen in these leads, but can be clearly seen in other leads.Morphology and amplitude:The P and QRS morphology both appear normal in shape. There are large R wave amplitude deflections in Lead I and the chest leads, consistent with voltage criteria for left ventricular hypertrophy (LVH). The ST segments in the infero-lateral leads, also show typical down-sloping "strain" pattern often seen with LVH. The P waves, where seen, are normal in duration (2-3 small squares) and height (<3mm high), and consistency (same appearance throughout). That all sounds like quite impressive banter, and not bad at all, considering you haven't even had to plump for a diagnosis yet! All you've done so far is 'describe what you see". So now let's concentrate on something else...Q5. What is the relationship between the P waves and the QRS? Can you calculate the PR interval? Does it alter? (Should be 3-5 small squares)Update 2The P-R interval does alter, making it impossible to calculate.As far as I can see the relationship is chaotic. (Has anyone else spotted any pattern that i've missed?)When there is no relationship between the atrial and ventricular depolaristions, they must occurring independently, from separate pace-maker sites. In other words, Atrioventricular dissociation is present.This ECG shows complete heart block. But what about the QRS Morphology? We've said it's normal. This ECG shows a narrow QRS complex escape rhythm. The QRS complexes often seen in ventricular 'escape' beats are broad, and oddly shaped in appearance. This is because of the abnormal spread of the depolarisation from the 'make-shift' safety-net pacemaker, as it misses the usual conductive tissue pathways.If, however, that abnormal pacemaker focus just happens to be very high, close to the original one, and is situated proximal to the bifurcation of the Bundle of His, then it is a junctional pacemaker and the QRS complexes will be close to normal morphology.The symptoms described by the patient are classical of Stoke-Adams Attacks and result from a sudden drop in cardiac output, and therefore cerebral perfusion. Recovery is rapid when the cardiac rhythm is restored. This patient is at high risk of sudden death and should be admitted for a permanant pacemaker insertion.

Complete heart block is a potentially life threatening arrhythmia. ABCDE guidelines should be instigated in a symptomatic patient and The Resuscitation Council UK 2010 Guidelines for "Adult Bradycardia" should be followed. In CHB the Sino-Atrial node functions normally so atrial contraction is normal, and P waves are present and normal.But...the electrical impulses are blocked at the Atrioventricular node, so NO beats are conducted to the ventricles. To compensate for this, in order to maintain cardiac output, the ventricles are excited by a 'slow escape' mechanism from a depolarising focus within the ventricular myocardium. As a result, in CHB:

There is no relationship between the P and the QRS waves.

The atrial rate is NORMAL

The Ventricular rate is SLOW.

If you ever see slow QRS complexes without associated P waves, you can generally assume a Ventricular escape rhythm.The QRS complex is often, but not always, abnormal in shape (for the reasons discussed above).

Causes of complete heart block

ACUTE. Myocardial infarction (MI). CHB may also be transiently seen during an MI.

CHRONIC.

Fibrosis or scarring around the Bundle of His

A block of BOTH Bundle branches at the same time (i.e. LBBB plus RBBB = CHB)

CHB versus Atrioventricular Dissociation (AV Dissociation)

As discussed above, CHB gives rise to a slow ventricular escape rhythm, and the atrial rate is faster than the ventricular rate.

AV Dissociation which is not due to a complete heart block, sometimes arises during an excessive sinus bradycardia. To compensate for long bradycardic pauses between beats, a faster 'escape' rhythm than the background sinus rate kicks in. This escape rhythm arises from the AV junction or ventricles. As a result, the ventricular rate is faster than the Atrial rate. ;-)